Abyssalite

Abyssalite is a very strong solid element in the Miscellaneous category and is found in large veins separating Biomes. It acts as a thermal insulator between biomes, preventing heat exchange between them as long as the abyssalite boundary remains intact. In order to mine abyssalite, a duplicant must possess the Superhard Digging skill. Abyssalite cannot be directly used as a construction material, but it can be combined with Reed Fiber and the Rare Resource Isoresin inside a Molecular Forge to create Insulation, which is an extremely similar material that can be used in construction.

Usage
Molecular Forge can produce Insulation from Abyssalite, Isoresin, Reed Fiber.
 * 15 kg Isoresin + 80 kg Abyssalite + 5 units Reed Fiber = 100 kg Insulation.

Abyss Bugs can eat Abyssalite.

Abyssalite will convert into tungsten when melted so if more thermium is needed in a colony, and it is low on tungsten then it is possible to melt it in a high temperature chamber.

Thickness of Abyssalite Veins
The aforementioned veins and seams of Abyssalite are generally two tiles thick, after the game world is generated. Two tiles are the minimum thickness that is necessary to lock in heat and cold, due to the mechanics of Thermal Conductivity.

At just one tile thick, Abyssalite veins will not do much because the materials on either side can impart and draw heat at a little over half of their own thermal conductivity.

Thus, when excavating Abyssalite, especially those that seal in Volcanic Biomes, care should be taken not to thin out the seams too much. It is good practice to check the Temperature Overlay when doing so. Because of Flaking, hot Abyssalite tiles can very quickly transfer heat, totally ignoring Abyssalite low thermal conductivity, to liquids dropped on them to make them change state.

Trivia
Abyssalite is named Katairite in game code.